Abstract
Abstract. Although the northwest margin of Australia is an important region for petroleum exploration and palaeoceanographic investigations, its Palaeogene stratigraphy is poorly documented, especially in terms of a foraminiferal biozonation. Early Palaeogene cores from 502.96 to 307.80 m below sea floor at Ocean Drilling Program Site 762 on the Exmouth Plateau were examined in this study for their planktic foraminiferal assemblages and the carbon isotopic compositions of Subbotina spp. Planktic foraminifera are generally well preserved and belong to 74 species and 17 genera. In spite of a mid-latitudinal palaeolatitude (c. 40°S) the sequence, deposited between the early Paleocene and Middle Eocene, contains all planktic foraminiferal Zones P1c through P10 of the current global scheme for tropical locations, except for Subzone P4b. Most zones are well defined by the datums of primary marker species except P3a and P9, which have boundaries that probably occur in core gaps, and the P9 zonal boundaries are defined by secondary marker species. Overall, variations in δ13C based on sequential samples of Subbotina are similar in pattern and magnitude to global summary isotope curves spanning the early Palaeogene. However, the prominent δ13C excursion that characterizes the Palaeocene/Eocene transition is mostly missing and appears to lie in a core gap. The planktic foraminiferal zonation, linked with that based on nannofossils, a recalibrated magnetostratigraphy and carbon isotope records, provides a robust temporal framework for the Early Palaeogene of the northwest margin of Australia.
Highlights
The Early Palaeogene c. 65 to 49 Ma (Fig. 1) is an especially significant interval of time because of profound changes in climate, ecosystems and the global carbon cycle (e.g. Zachos et al, 1993, 2001; Corfield, 1994; Thomas & Shackleton, 1996; Dickens et al, 1997; Norris & Röhl, 1999)
The timescale used in previous work (Shipboard Scientific Party, 1990) is outdated, precluding direct age comparisons with other regions. This study addresses these deficiencies by examining planktic foraminifera and their isotopic composition in sediment from Ocean Drilling Program (ODP) Site 762
Foraminiferal biostratigraphy Planktic foraminifera contribute between 10% and 20% of bulk sample volume
Summary
The Early Palaeogene c. 65 to 49 Ma (Fig. 1) is an especially significant interval of time because of profound changes in climate, ecosystems and the global carbon cycle (e.g. Zachos et al, 1993, 2001; Corfield, 1994; Thomas & Shackleton, 1996; Dickens et al, 1997; Norris & Röhl, 1999). To resolve outstanding questions of global change, early Palaeogene marine successions must be located that meet certain criteria. These targets must be: (1) thick so as to maximize the temporal quality of the record and time resolution; (2) buried with little overburden to reduce the logistical demands of drilling and diagenetic effects; (3) rich in well-preserved biogenic carbonate for geochemical evaluation; and (4) hosted in stratal geometries which permit construction of detailed depth and latitudinal transects. Studies of this region have already advanced our knowledge of Palaeogene environmental change significantly Studies of this region have already advanced our knowledge of Palaeogene environmental change significantly (e.g. Bains et al, 1999; Katz et al, 1999; Norris et al, 1999; Norris & Röhl, 1999)
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